Because you are putting it on Adobe Servers when you Publish online. You need to sign in with your Adobe ID. Just don't use that service if you do not want to sign-in. Adobe like Microsoft, Apple and all the rest now provide cloud computing, storage & services. You need to sign into the services to use it. Some organizations like the Federal Government do not let their users use the cloud solutions. It is an optional service.
Publish Online requires at least InDesign CC 2015 or later. There's no way it can work with InDesign CS6, sorry.
If you are on CS6 then you are a creative suite member and not a CC creative cloud member.
You need CC 2015 or later to try Publish online.
Hmm. I thought it came out in CC. Could be wrong... But yes, you need to in the cloud.